A. The Fusion Process
The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. See, for example, Varshneya, Arun K., “Flat Glass,” Fundamentals of Inorganic Glasses, Academic Press, Inc., Boston, 1994, Chapter 20, Section 4.2., 534–540. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty, the contents of which are incorporated herein by reference. A schematic drawing of the process of these patents is shown In FIG. 1. As illustrated therein, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a refractory body 13 known as an “isopipe.”
Once steady state operation has been achieved, molten glass passes from the supply pipe to the trough and then overflows the top of the trough on both sides, thus forming two sheets of glass that flow downward and then inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root 15 of the isopipe, where they fuse together into a single sheet. The single sheet is then fed to drawing equipment (represented schematically by arrows 17), which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located well downstream of the root so that the single sheet has cooled and become rigid before coming into contact with the equipment.
As can be seen in FIG. 1, the outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
In a commercial setting, the location and inclination of isopipe 13 needs to be adjustable with respect to the equipment used to melt and refine the raw ingredients from which the glass sheet is made. Although theoretically such adjustability could be achieved in a system in which the connection between the melter/finer and the fusion system was completed closed (i.e., a system in which entire feed system to the isopipe was filled with molten glass), in practice, adjustability is achieved by connecting the melter/finer to the fusion system by forming a free surface of molten glass.
FIGS. 2 and 3 illustrate such a connection, where as above, 9 is the supply pipe to isopipe 13, 19 is the exit from the melter/finer system (referred to herein as a “downcomer” since it preferably has a substantially vertical (downward) orientation), arrow 22 shows the direction of flow of molten glass 31, and 21 is the free surface of the molten glass, the height of which relative to the height of molten glass in the isopipe's trough 11 is determined by (1) the rate of flow of molten glass out of the downcomer and (2) the resistance to fluid flow of the supply pipe/isopipe combination. As can be seen in these figures, because the characteristic cross-sectional dimension of downcomer 19 (e.g., the diameter of the downcomer) is smaller than the characteristic cross-sectional dimension of supply pipe 9 (e.g., the diameter of the entrance 18 to the supply pipe), the downcomer and supply pipe can be readily moved relative to one another. In this way, the desired adjustability between the melter/finer and the fusion system is achieved.
It should be noted that for a downcomer whose exit end 20 is submerged in molten glass, the height of free surface 21 relative to the height of molten glass in trough 11 is relatively insensitive to changes in the depth of submersion of the exit end. To provide a spatial reference for describing the invention, the phrase “nominal free surface” is used herein to indicate the location of free surface 21 when exit end 20 is just submerged in the molten glass. The reference number 21N is used to identify the nominal free surface.
For essentially any practical submersion of the exit end of the downcomer, the nominal free surface and the actual free surface will be at essentially the same location. Accordingly, in FIGS. 2 and 3 both reference number 21 and reference number 21N are used to identify the interface between molten glass 31 and the surrounding atmosphere 33 (typically air).
It should also be noted that because supply pipe 9 (as well as downcomer 19) are made of opaque refractory materials (e.g., platinum or a platinum alloy), neither the actual free surface nor the nominal free surface of the molten glass can be visually observed. However, their locations can be accurately estimated using physical modeling (e.g., oil modeling). In this connection, it should be noted that the free surfaces 21 shown in the figures and, in particular, in FIGS. 8–9 and 11–12, are simplified drawings for purposes of illustration, it being understood that the actual free surfaces will have more complex shapes as a result of the molten glass transitioning from a smaller diameter conduit to a larger diameter conduit at a free surface. Further, knowledge of the exact locations/configurations of the actual free surface and the nominal free surface is not needed to practice the invention since, as explained in detail below, by examining the finished glass for defects, specifically, for devitrification and blister defects, one can determine if the spatial relationship between the downcomer and the molten glass is within the operative range defined by the invention.
B. LCD Glasses
Corning Incorporated, the assignee of this application, has sold glass sheets for use as substrates in the manufacture of liquid crystal displays under the trademarks 1737 and EAGLE 2000. See, U.S. Pat. No. 5,374,595 to Dumbaugh, Jr. et al. and U.S. Pat. No. 6,319,867 to Chacon et al., respectively, the relevant portions of which are incorporated herein by reference.
EAGLE 2000 glass has a silica content of approximately 63.3 wt. %, while 1737 glass has a silica content of approximately 57.8 wt. %. Because of its higher silica content, EAGLE 2000 has a greater tendency to devitrify than 1737, e.g., to form cristobalite, the high temperature crystalline form of silica.
To address EAGLE 2000's greater tendency to devitrify, its formulation includes a higher percentage of boron oxide (B2O3), specifically, approximately 10.3 wt. % B2O3 for EAGLE 2000 versus approximately 8.4 wt. % for 1737.
Notwithstanding this higher level of B2O3, during trial manufacturing runs, considerable cristobalite devitrification was observed when EAGLE 2000 glass was manufactured using equipment which previously had successfully produced 1737 glass without the generation of high levels of devitrification. The devitrification of EAGLE 2000 glass was first observed in the compression beads at the edges of the glass sheet (i.e., the beads engaged by the drawing equipment) and eventually throughout the glass sheet, including the quality portion of the sheet intended ultimately to form the LCD substrate.
The present invention is concerned with identifying the source of this devitrification and with providing methods and apparatus for eliminating this defect without introducing other defects (specifically, blister defects) into the finished glass sheets.